This invention relates generally to gas turbine components, and more particularly to turbine airfoils.
A gas turbine engine includes a compressor that provides pressurized air to a combustor wherein the air is mixed with fuel and ignited for generating hot combustion gases. These gases flow downstream to one or more turbines that extract energy therefrom to power the compressor and provide useful work such as powering an aircraft in flight. In a turbofan engine, which typically includes a fan placed at the front of the core engine, a high pressure turbine powers the compressor of the core engine. A low pressure turbine is disposed downstream from the high pressure turbine for powering the fan. Each turbine stage commonly includes a stationary turbine nozzle followed by a turbine rotor.
The turbine nozzle comprises a row of circumferentially side-by-side nozzle segments each including one or more stationary airfoil-shaped vanes mounted between inner and outer band segments defining platforms for channeling the hot gas stream into the turbine rotor. Each of the vanes includes pressure and suction sidewalls that are connected at a leading edge and a trailing edge. The airfoil section typically has a broad, blunt leading edge having a region of high curvature referred to as a “high C” point transitioning from the leading edge to a convex shaped suction surface.
The thermal stress at the suction side high C point can be too high under the operating conditions. The gas flowfield in the platform region is very complex and highly three dimensional. When gas flow approaches the turbine blade leading edge, at the junction of leading edge and platform, there is a total pressure gradient in the radial direction within the boundary layer that results in the formation of a pair of counter-rotating horseshoe vortices on pressure and suction sides near the platform. The pressure side horseshoe vortex travels along the pressure gradient on the platform between adjacent blades and forms a pressure side passage vortex. The suction side horseshoe vortex travels along the suction side surface and migrates upward toward the trailing edge to form a suction side passage vortex. Both pressure and suction side passage vortices interact near the suction side trailing edge at an upper span location and cause total pressure loss and reduce turbine efficiency. The horseshoe vortices and passage vortices also create more turbulence and increase the heating to the platform surface.
Accordingly, there is a need for a turbine nozzle having low stresses at the junctures between the airfoil body and the attached platforms.